The first one's not strictly a Discworld quote, but as a gamer it sort of appeals to me.

Over the centuries, mankind has tried many ways of combating the forces of evil... prayer, fasting, good works and so on. Up until Doom, no one seemed to have thought about the double-barrel shotgun. Eat leaden death, demon...

Terry Pratchett - alt.fan.pratchett (30 May 1998)

Look, that's why there's rules, understand? So that you think before you break 'em.

Terry Pratchett - Thief of Time

Sam Vimes could parallel process. Most husbands can. They learn to follow their own line of thought while at the same time listening to what their wives say. And the listening is important, because at any time they could be challenged and must be ready to quote the last sentence in full. A vital additional skill is being able to scan the dialogue for telltale phrases such as "and they can deliver it tomorrow" or "so I've invited them for dinner?" or "they can do it in blue, really quite cheaply."

Terry Pratchett - The Fifth Elephant

People who didn't need people needed people around to know that they were the kind of people who didn't need people.

'Lu-Tze looked into those eyes. After 800 years you learn how to read people. And Susan was a story that went back a very long way. She probably even knew about Rule One, and didn't care.'
Thief of Time